Even brief study of the prior art relating to medical electrical leads will reveal that such leads typically incorporate at least one electrode and the lead assembly should be compact and resilient, should yield a low threshold for stimulation, should sense the low amplitude electrical signals naturally generated by a body. In addition, such leads should be biocompatible with the adjacent body tissue and body fluids, which it contacts. Various attempts have been made to improve these characteristics, especially with respect to medical electrodes electrically coupled to the lead body of a cardiac pacing lead. Generally these attempts are aimed at increasing the interfacial, or active, surface area of an electrode by the use of surface treatments or coatings that are considered highly biocompatible.
As is known in the art, electrochemical reactions occur at the electrode-tissue interface when an electrical stimulation pulse is delivered through a medical electrical lead assembly. This phenomenon is referred to as “polarization,” which is known to interfere with efficient delivery of the stimulation pulses. High interfacial impedance due to the effects of polarization reduces the effective charge transfer of the stimulation pulse to the targeted tissue. Therefore low polarization electrodes have been developed to reduce this effect and improve the transfer of charge from the electrode to the tissue.
One method for reducing polarization effects is to increase electrode surface area. However, a design trade-off exists in increasing the electrode size since medical leads and the electrodes they carry are preferably of small dimensions such that they may be easily implanted. For example, presently available cardiac pacing leads typically have cross-sectional diameters of greater than about three or four French and a typical diameter of an electrophysiology catheter is about six French. For reference, a single French unit of measurement equals one third of a millimeter. In any event, to overcome this trade-off, methods for increasing the active surface area of a geometrically down-sized electrode have been proposed. For example, treatments or coatings that yield a porous, irregular or grooved surface increase the active surface area of the electrode and thereby diminish the effects of polarization. Various coatings have been proposed and put into commercial use for producing low polarization electrodes such as platinum black, pyrolytic carbon, iridium oxide, vitreous carbon, titanium nitride and others.
A further benefit of increasing electrode interfacial or active surface area can be improved electrode sensing performance. Cardiac signals, including action potentials that are relatively low amplitude, monophasic signals, may be more readily sensed when the active surface area of the electrode is increased. Moreover, an evoked response following delivery of a stimulation pulse may be more readily detected when post-pace polarization artifact is diminished.
Recently, as reported in the Journal of the American Chemical Society (JACS), research personnel at Washington University in St. Louis and their collaborators report that they have made boron “nanowhiskers” by chemical vapor deposition. The particles have diameters in the range of 20 to 200 nanometers and the whiskers (also called nanowires) are semiconducting and show properties of elemental boron.
The group at Washington University in St. Louis turned to boron, one spot to the left of carbon in the periodic table, to see if it would be a good candidate. They postulated that if nanotubes could be made of boron and produced in large quantities, they should have the advantage of having consistent properties despite individual variation in diameter and wall structure. The discovery that the “nanowhiskers” are semiconducting make them promising candidates for nanoscale electronic wires. Boron nitride nanotubes, which are similar in structure to carbon nanotubes, are electrically insulating. Boron nanotubes on the other hand may be grown into long thin wire-like structures. At first they appeared hollow, but after closer examination, they were determined to be dense whisker-like structures, not hollow nanotube structures. The notion of boron nanotubes creates more excitement in nanotechnology than nanowhiskers because of their unique structure, which could be likened to a distinct form of an element. Carbon, for instance, is present as graphite and diamond, and, recently discovered, in “buckyball” and nanotube conformations. Also, boron nanotubes are predicted by theory to have very high conductivity especially when bulk boron is “doped” with other atoms to increase conductivity. Carbon nanotubes also have been doped, as have various other kinds of nanowires, and assembled in combinations of conducting and semiconducting ones to make for several different microscale electronic components such as rectifiers, field-effect transistors and diodes.